Have you ever noticed something swimming in your field of vision?
你是否会注意到自己的视线中有漂浮物？
It may look like a tiny worm or a transparent blob, and whenever you try to get a closer look,
它看起来像是小虫或者透明液滴，但当你想要更仔细地看它的时候
it disappears, only to reappear as soon as you shift your glance.
它又消失了转换视线之后它又会出现。
But don't go rinsing out your eyes!
但你别去冲洗眼睛！
What you are seeing is a common phenomenon known as a floater.
你看的其实是一种常见现象，飞蚊症。
The scientific name for these objects is Muscae volitantes, Latin for "flying flies,"
科学界称之为Muscae volitantes，是拉丁语中飞蚊的意思
and true to their name, they can be somewhat annoying.
物如其名，它们有时很讨厌。
But they're not actually bugs or any kind of external objects at all.
但它们不是真正的虫或者身外之物。
Rather, they exist inside your eyeball.
它们就存在于你的眼球中。
Floaters may seem to be alive, since they move and change shape, but they are not alive.
因为“飞蚊”会不断地变形，所以人们会觉得它们是有生命的其实不是。
Floaters are tiny objects that cast shadows on the retina, the light-sensitive tissue at the back of your eye.
“飞蚊”是一种极小的物质，投影于视网膜也就是眼球后部的光敏组织。
They might be bits of tissue, red blood cells, or clumps of protein.
它们可能是小块组织，红细胞或是蛋白质团。
And because they're suspended within the vitreous humor, the gel-like liquid that fills the inside of your eye,
由于它们悬浮于玻璃体，也就是眼内的胶状组织“飞蚊”，
floaters drift along with your eye movements, and seem to bounce a little when your eye stops.
会随眼球而移动，眼球停止转动时它又会少许地弹动。
Floaters may be only barely distinguishable most of the time.
“飞蚊”大多数时候不易被察觉。
They become more visible the closer they are to the retina,
当它们靠近视网膜时，它们变得可看见
just as holding your hand closer to a table with an overhead light will result in a more sharply defined shadow.
这就相当于你把手放在吊灯下更接近桌子时手的阴影会更明显。

And floaters are particularly noticeable when you are looking at a uniform bright surface,
“飞蚊”也会很明显比如当你盯着发光的表面时，
like a blank computer screen, snow, or a clear sky, where the consistency of the background makes them easier to distinguish.
像是空白的屏幕雪或是晴空，因为颜色一致的背景会使“飞蚊”更明显。
The brighter the light is, the more your pupil contracts.
当看到的光越亮瞳孔会收缩的更厉害。
This has an effect similar to replacing a large diffuse light fixture with a single overhead light bulb,
这就好比把许多散射的光换成一个大的聚光灯
which also makes the shadow appear clearer.
也会使得阴影更加明显。
There is another visual phenomenon that looks similar to floaters but is in fact unrelated.
另外也有一种视觉现象与飞蚊症类似但与之无关。
If you've seen tiny dots of light darting about when looking at a bright blue sky,
当你仰望蓝天时会看到小光点在快速闪动，
you've experienced what is known as the blue field entoptic phenomenon.
这被称之为谢瑞尔氏现象。
In some ways, this is the opposite of seeing floaters.
这与看到飞蚊相反。
Here, you are not seeing shadows but little moving windows letting light through to your retina.
此时，你看到的不是阴影，而是一些移动的小开口让光进入视网膜。
The windows are actually caused by white blood cells moving through the capillaries along your retina's surface.
这些开口是由白细胞造成的白细胞穿梭在视网膜表面的毛细血管。
These leukocytes can be so large that they nearly fill a capillary causing a plasma space to open up in front of them.
白细胞可以大到填满整个毛细血管，这导致了白细胞身前的空隙。
Because the space and the white blood cells are both more transparent to blue light than the red blood cells normally present in capillaries,
由于这些空隙和白细胞本身相比于血管中的红细胞更能让蓝光透过我们，
we see a moving dot of light wherever this happens, following the paths of your capillaries and moving in time with your pulse.
因此看到小光点沿着毛细血管移动也随着脉搏而移动。
Under ideal viewing conditions, you might even see what looks like a dark tail following the dot.
在理想状况下甚至会看到光点后有条黑尾巴。
This is the red blood cells that have bunched up behind the leukocyte.
这是白细胞身后堆积的红细胞造成的。
Some science museums have an exhibit which consists of a screen of blue light,
有些科技馆中会展览出一整屏幕的蓝光
allowing you to see these blue sky sprites much more clearly than you normally would.
这会让你跟清楚的看到这一现象。
While everybody's eyes experience these sort of effects, the number and type vary greatly.
因为虽然每个人都会经历这些现象，每个人看到的程度都不同。
In the case of floaters, they often go unnoticed, as our brain learns to ignore them.
就飞蚊症而言大多数会被我们的大脑所忽视。
However, abnormally numerous or large floaters that interfere with vision
但是数目异常多或者大到影响视线的飞蚊，
may be a sign of a more serious condition, requiring immediate medical treatment.
可能意味着你需要紧急医疗救助。
But the majority of the time entoptic phenomena, such as floaters and blue sky sprites, are just a gentle reminder that
但是多数的眼内现象，比如飞蚊或者谢瑞尔氏现象，只是委婉地提醒我们
what we think we see depends just as much on our biology and minds as it does on the external world.
自认为看到的东西取决于我们的生理和大脑，就和取决于外部世界一样。